In a present day projector, a built-in AC/DC power converter is most prevailing. As shown in FIG. 1, power supply 11 and ballast 12 are set up inside the casing 10 of projector. The power supply 11 is connected to an external power cord 112 of the projector. The grid power of 110 VAC or 220 VAC is introduced to the power supply 11 by an external plug 111 and is converted to different voltages required by the projector through the circuit framework of the power supply 11 and the circuit framework of the ballast 12. Please refer to FIG. 2, which illustrates the circuit framework of the power supply 11 and the circuit framework of the ballast 12 according to the prior arts. The power supply 11 is a power factor refining circuit (PFC), by which the grid power of 110 VAC or 220 VAC can be transformed into a high voltage of 380 volts and thereafter output to both the ballast 12 and a sub-power circuit 13 which is required by the circuit board. The ballast 12 applies a step-down circuit to transform the 380 volts voltage into the voltage (about 80 volts) needed by the lamp 121. The sub-power circuit 13 applies a fly-back circuit framework to transform the 380 volts voltage into different voltages (such as: 12 volts, 5 volts, and 3.3 volts, etc.) needed by the circuit board, wherein a transformer T1 and an opto-coupler IC1 are needed by the sub-power circuit 13 to isolate the first side and the second side.
According to the national security test standard and the consideration of safety factor, an exposed voltage can not exceed 60 volts while the voltage generated in the power supply of a projector must reach 380 volts to light up the lamp. Therefore, this is the main reason why the power supply device must be built in a traditional projector. Because of the specific space occupied by power supply that is set up in the projector, the volume of the projector can not be reduced. Hence, to move the power supply out of the projector has become an arduous problem for current manufacturers.